The Universe, Black Holes and Entropy

I was just reading an article about the asymmetry of time ( and it mentions that the initial conditions of the universe were low entropy, and that a black hole is high entropy.

They both seem like lots of matter in a small space so I would expect them to be similar with respect to entropy. Is the difference that spacetime itself was significantly different at the initial conditions of the universe?

By definition, black holes form in the wake of the gravitational collapse of supernovae, which are very massive stars that ran their fuel cycle.

But their relative short lifespan compared to average stars like our Sun is still measured in the hundreds of millions if not low billions of years, so if the Big Bang is correct, those stars were just being born and black holes could not have existed until many moons after the initial “Bang”.

Edit: I realize you weren’t asking for that information, but I was thinking aloud, at least as aloud as an internet messageboard allows for.

Is it the difference between a black hole and an otherwise empty universe containing a black hole? A big region of empty space or a small region of high density matter are both very uniform, but a big region of otherwise empty space that also contains a smaller region of high density matter is not so uniform.

FoiGras, I think a black hole is defined by its physical nature, not by its origin. Typically, stellar sized black holes form from collapsing stars, but not necessarily sypermovae - more typically, I think (?), stellar sized holes form from remnants that grow cool enough not to be able to sustain their diameter through thermal resistance to contraction.

If I understand the OP correctly, he’s asking why the very young universe wasn’t a black hole. I’d like to understand that myself.

Yes, I think that’s basically the same question, I think.

Hmm… not an astronomer or astrophysics guy, but I suspect it has to with heat and expansion.

The early universe was hot and expanding - it had started from a dense state, but everything was flying out and cold empty spaces developed soon. That gave you a lot of options open to transfer energy from hotspots to cold areas, which is low entropy as I understand it. There’s a lot of thermal transfers you can still do without violating the law about transfering from a cold body to a hot body.

A black hole, though, is not really hot or cold as such, and it’s not expanding. Because everything is squashed together, there’s no opportunity to do any thermal transfers at all, so it’s very high entropy.

If we had a way to make black holes ‘bang’ on cue and expand in a mass of hot matter, then we’d be reducing the entropy of the universe on a local scale, as I understand it.

It wasn’t all that long ago (a decade or so) that the possibility that the Universe was (and hence still is) a black hole was taken very seriously. You’ve heard of the question of whether the Universe will expand forever, or re-collapse in a “big crunch”? Well, a Big Crunch universe is a black hole (the crunch singularity at the end is the center of the hole). This possibility is no longer considered likely, thanks to newer measurements, and it appears that what keeps the Universe from re-collapsing is primarily the dark energy, or cosmological constant. Without going into too much detail, suffice to say that the presence of a significant density of dark energy can cause something to not be a black hole which otherwise would be, or cause a black hole to have different properties.

There’s another difference yet between a black hole and the big bang. A black hole is a compression of matter within space; the big bang was an expansion of matter and space. Since entropy may without loss of generality be described in terms of the organization of matter and energy, the formation of the black hole doesn’t change the entropy in nearly the same way as the initiation of the observable Universe.

A lot of matter in a small space can be either high- or low-entropy depending on the circumstances.

For example, consider a sealed vessel containing a gas. Having all the molecules in the gas clumped at one end of the vessel is a low-entropy state. Having all the molecules spread evenly through the vesseal is a high-entropy state.

But consider an amount of gas large enough to form a star. Then having all the molecules spread out in a diffuse cloud is a low-entropy state. Having them all clumped together in a black hole is a high-entropy state.

The difference is that in first case the effects of gravity are minimal. And in the second they dominate.

The early universe contained enough matter for gravity to be an important factor in its behavior. And yet it was very homogenous – there was very little clumping. That means it started out in a low-entropy state.

Black holes are areas of maximal local entropy; that is, they have maximum entropy for the (nominal) volume they occupy. Why? Because they are absorbing matter or energy that has a finite amount of entropy but compressing it into a fundamentally minimal volume (as compact as it is possible for energy to exist). This is in conflict with a classical view of thermodynamics, in which the absorption of all associated matter and energy is complete, i.e. there is “no hair”, no way to tell what is in within the event horizon of a black hole because no particles (and thus, no information) can escape, and no low temperature reservoir to which to provide entropy balance. This is resolved by permitting a black hole to radiate away a certain amount of energy (from which comes the quantum electrodynamic field theory-based “Hawking radiation”) at a specific temperature based strictly on its mass, electric charge, and rotational momentum, so that it can maintain thermodynamic balance and still behave as General Relativity describes. (A black hole can essentially be treated as single a giant composite quantum particle insofar as it can be distinguished only by these limited properties.) Stellar mass black holes actually radiate energy at a lower temperature than the surrounding background, and so are themselves electromagnetically invisible from any distance, though the behavior of matter surrounding and infalling into them can give signiture traces in the form of synchnotron radiation.

Actually, homogeneity of energy is characteristic of a high entropy state. The early universe enjoyed low entropy by being so compressed in volume–and with such a vast amount of unbound energy–that even very tiny anisotropies gave low entropy. As the universe expanded symmetries between the fundamental forces broke, and as matter and energy condensed into observable forms these fine differences led to large variations in thermal and gravitational equilibrium, which in turn formed the non-fundamental celestial structures (from stars to galactic superclusters) we see today, and eventually, higher elements and rocky bodies like the Earth.

Although it is common to think of the early universe as being a big singularity, the fact is that the space it occupied was also singular; unlike a black hole, it had no external place to radiate to. The model of a black hole is then turned on its head, as if the universe is a black hole in reverse or inside out. Could we be inside of an expanding black hole which was conceived at the Big Bang? It has been suggested, though by no means widely accepted or falsifiable, but the implications are pretty meaningless, even for theorists. Just as we cannot see into the event horizon of a black hole, the occupants of a black hole could not see outside or obtain useful information across that boundary. We couldn’t even tell if the basic physical constants, and thus the behavior and extent of force interactions, would be the same.

Dark energy is basically a placeholder for a phenomenon which mathematically satisfies empirical observations but has no real explanation. We know, or at least think that we know, that there is a repulsive action that works upon spacetime in opposition to normal gravity, either as a previously unknown long range component of gravity, or some other fundamental force interaction that causes spacetime to expand. We don’t actually know anything about the underlying nature of it, though, only that it is needed in some form to copy with the Hubble Flow and to give instantiation to a stable or accelerating expanding universe.

Personally, I find the term “dark energy” to be far to prosaic and prefer “crepuscular palpitation” instead. Plus, then people look at me funny, like I’m describing an unspeakable medical procedure, which is a great way to keep a couch to yourself at a crowded party.


Why not? (I’m not challenging you; I’m just curious). Wouldn’t they be able to see light falling into the black hole?

I need to clarify; you wouldn’t be able to make sense of anything beyond your subjective horizon, which is within the absolute event horizon from an inertial observer outside the black hole. Radially incoming light would be blue-shifted to beyond gamma radiation to an observer who is approaching his apparent horizon (that is, the surface from the subject’s position at which light directed outward has an ultimate inward trajectory) while light with a tangential component will whip around a few times in spiral orbit into illegibility; not only does this have the obvious implications (i.e. you’d be well advised to invest heavily in leaded protective clothing) but you won’t be able to distinguish between light sources as the wavelengths of all frequencies shorten to indeterminacy. Because time slows down for you (relative to a non-accelerated external observer) as you approach the singularity surface, this all happens in a flash; even for a galactic mass (non-rotating) black hole your objective time from the apparent horizon to the singularity is only a few seconds, and subjective time is virtually instantanous. For rotating black holes things are a little more challenging, and with enough spin and just the right trajectory you could maintain an almost stable orbit inside of the event horizon, but you’re still not going to be able to make anything out. For all practical purposes it is a double blind.

The only good way to really get inside of a black hole is to get in on the ground floor when it is forming and the tidal and shearing stresses aren’t too great. Then maybe you can explore a new universe (or at least a pinched off area of this universe); but make sure you pack everything you need, because there’s no turning around for a forgotten toothbrush.


In fact, it’s actually possible to find yourself inside a black hole with no indication whatsoever that that’s the case, with all measurements normal to the umpteenth decimal place until you all of a sudden hit the singularity and everything goes to Hell. This could be happening to us, for all we can tell: We might some day (or some minute) just all of a sudden run into a black hole singu

I’m still trying to digest your post Stranger, but thanks all for the info

You really have to watch those left turns.

Here is a somewhat old but pretty good explanation (with graphics and animations) of what is experienced by the subjective and external objective observers with regard to dynamics in and around a black hole. Of course, once you get really, really close to the surface of the singularity, the math of General Relativity stops giving you meaningful results, indicating that either the underlying phenomena are even stranger than dreamt of in our natural philosophy (i.e. requiring some kind of theory of quantum gravity), or that we need some kind of transfinite mathematics and a lot of aspirin to cope with the entire business.